You should connect with your tax expert.
No one can tell you that without more information.
In general, if this was a tax-deferred account, like a traditional IRA, then you owe income tax, the same as if the owner had withdrawn the money. The broker would send you a 1099-R at the end of the year.
If the account held regular investments that were not tax-advantaged, then you inherited those investments on the date your mother died at the value on that date. If the investments were later sold (cashed out) you might have a taxable gain or a deductible loss. The broker would send you a 1099-B at the end of the year.
The death benefit on a life insurance policy is not taxable.
There are some other, more complicated investments that might be partially taxable.
I suggest you start by asking Prudential. If you are a subscriber to Turbotax Live, start by asking your Live expert and they can tell you what questions you need to ask Prudential.